My observations from experience and reading. It seems that for now, ranks do not really matter or make much of a difference at all for archery-based units. Gunpowder units most definitely benefit from 2 rank formation, increasing it will drastically lower their kills and kill rates. In general deeper ranks of spears should work better against cav, but there are enough "broken" units as of right now that cav pretty much mows down anything spear-based except braced pikes head on. Cav works best in wide to box type formation to maximize the impact in a charge.

Cheers